1. Field of the Invention
This invention generally relates to techniques of surface acoustic wave devices, and more particularly, to a technique that enables to enhance the power durability, lower the resistance, and lower the cost of a surface acoustic wave duplexer.
2. Description of the Related Art
A surface acoustic wave device as represented by a surface acoustic wave (hereinafter referred to as SAW) filter is widely employed for a mobile communication device such as a mobile telephone or the like. Particularly, in recent years, a dielectric element on an antenna duplexer has been increasingly replaced by the SAW device, for the purpose of downsizing and making the mobile communication device thinner. The antenna duplexer is used for separating transmission signals from reception signals. A great power is applied to the antenna duplexer, and accordingly it is considered essential that the SAW device used in the antenna duplexer has high power durability. The development has been made in order to enhance the power durability of the SAW device.
If an excessive power is applied to an interdigital transducer (IDT) that excites or receives a surface acoustic wave, a stress migration and an electromigration will be caused in an electrode of the IDT. The electrode is damaged by the migrations depending on the electrode material. This degrades the characteristics of the SAW device. Therefore, it can be said that the power durability of the SAW device is largely dependent on the material of the electrodes in the IDT. Under the above-mentioned background, the development of the electrode material for the SAW device has been made actively.
The results of the development of the conventional electrode material are classified as follows, according to the principle thereof. In a first method, a metal other than Al is added to Al in order to enhance the strength of the electrode material (refer to Japanese Patent Application Publication No. 62-163408 and Japanese Patent Application Publication No. 64-80113). In this method, a metal element such as Cu, Ti, Ni, Mg, Pd, or the like that can improve the migration resistance characteristics (power durability) is added to Al of the main electrode material in order to enhance the strength of the electrode material.
In a second method, the crystal grain size of Al, which is the electrode material, is reduced to produce a large number of crystal grain boundaries so that the stress applied to the electrode may be released structurally (refer to Japanese Patent Application Publication No. 8-32404 and Japanese Patent Application Publication No. 8-148966). In this method, two or three given elements are added to Al of the main electrode material to reduce the size of the grains and produce a large number of the crystal grain boundaries so that the stress may be dispersed for improving the power durability.
In a third method, a double-layered structure having an electrode layer of Al provided on a thin underlying film is employed so that the crystalline orientation of the electrode layer of Al may be made in one direction, or a single crystal of Al is employed. The mechanical strength of the electrode is thus enhanced (refer to Japanese Patent Application Publication No. 5-90268, Japanese Patent Application Publication No. 10-93368, and International Publication WO99/16168). Specifically, a thin film of Ti or Cr is formed on a piezoelectric substrate, and then the electrode layer of Al, which is the main electrode material, is formed to be oriented in a certain crystal orientation in order to enhance the power durability of the electrode layer of Al.
In a fourth method, the electrode is configured to employ a laminated structure having three or more layers in order to enhance the strength of the electrode (refer to Japanese Patent Application Publication No. 4-288718, Japanese Patent Application Publication No. 9-223944, Japanese Patent Application Publication No. 7-122961, U.S. Pat. No. 5,909,156, Reissue Application No. 38278, and Reissue Application No. 38002). Specifically, a multilayered electrode structure having three or more layers is employed to include the underlying film, a film having a main component of Al, and a metal film other than Al. The mechanical strength is thus improved.
Today, however, it is difficult to satisfy the characteristic demands for the SAW device (mainly three points, to enhance the power durability, lower the resistance, and lower the costs) with the above-mentioned first through fourth methods. A description will be given of the reasons. The conventional frequency band of the SAW device has been mainly 800 MHz to 1 GHz. However, the frequency band of 1.8 to 2 GHz is mainly used these days. This means that the frequency band becomes almost double these days. This reduces the width of the electrode finger and the gap between the electrode fingers of the IDT to half or less of the conventional one, and also reduces the thickness of the electrode film to half or less. As the electrode finger becomes smaller, the power durability is degraded. Therefore, the power durability of the electrode material itself has to be enhanced more. In addition, the width of the electrode finger and the thickness of the electrode film are respectively reduced to half or less, and a cross-sectional area of the electrode becomes one forth or less and an electric resistance thereof is accordingly increased four times or more. If the electric resistance is increased, the pass band characteristics of the SAW device will be degraded. Therefore, it is necessary to avoid the increase in the electric resistance as much as possible. Moreover, the spread of the mobile communication devices promotes to lower the cost thereof, and accordingly the cost of the SAW device has to be lowered. Here, a description will be given of the studies that have been made whether the above-mentioned first through fourth methods are capable of satisfying the above-mentioned demands.
First, the electrode has a single-layered structure in the first and second methods. A substantial amount of a metallic element other than Al has to be added to Al of the main electrode material in order to obtain sufficient power durability. However, thus added metallic element results in an increase in the electric resistance.
In addition, with respect to the third method, the sufficient power durability is not obtainable by only providing the Al film having the high orientation on the extremely thin underlying film. As described in Japanese Patent Application Publication No. 2002-368568, the characteristic demands can be satisfied with only a thick underlying film, which is formed to have a high film thickness ratio to the whole electrode. However, an electrical resistivity of Ti, which is often used for the underlying film, is approximately 20 times higher than that of Al, the main electrode material. If the Ti underlying film is thick, the electrical resistivity of the electrode will be increased.
Further, the fourth method employs the multilayered structure having three or more layers. This makes an electrode forming process (such as film forming and etching) complicated and increases the cost. Also, the ratio of Al of the main electrode material having a relatively low resistance is decreased, which increases the electrical resistivity of the electrode.
As described, it is difficult to provide the SAW device that can satisfy the demands of enhancing power durability, lowering the resistance, and lowering the cost all together, with the conventional techniques.